Translation: Eukaryotes

Initiation of Translation

• 40S subunit interacts with eIF1, 1A and 3.
• Prevents small ribosomal subunit from binding to the large subunit

1. [The 40S subunit with eIF1, 1A and 3] interacts with [eIF5] and [tRNAiMet with eIF2:GTP]
2. If eIF2:GDP is phosphorylated, it can not bind tRNAiMet and translation is blocked
3. The mRNA is bound by eIF4, which interacts with both the 5'cap and the 3' poly(A) tail protein PABPC1
4. This leads to a CIRCULAR mRNA structure
5. Forms a scaffold for other eIF4 subunits
6. eIF4G interacts with eIF3
7. The complex moves along the mRNA (5'--> 3') until it finds a start site (AUG)
8. eIF4A (stimulated by eIF4B) has helicase activity which unwinds the RNA's secondary structure
9. eIF2:GTP is irreversibly hydrolyzed to eIF2:GDP stop scanning
10. Facilitatied by eIF5, a GTPase activating protein
11. eIF5B:GTP mediates the large ribosomal subunit associating with the small subunit
12. eIF5B:GTP is hydrolyzed to eIF5B:GDP
13. eIF5B:GDP and eIF1A are released from the mRNA

1. IRES forms a complex with eIF4A and eIF4G
2. These associate with the 40S subunit, eIF1, 1A and 3
3. These associate with eIF2 and tRNAiMet

Elongation in Translation

1. After initiation, the tRNAiMet is positioned at the P site on the ribosome
2. The second amino acid is brought in to the A site on an aminoacyl-tRNA bound to EF1$a$:GTP (elongation factor)
3. When the tRNA carrying the correct amino acid enters the A site (as determined by anti-codon:codon pairing), EF1$a$:GTP is hydrolyzed to EF1$a$:GDP
4. EF1$a$:GDP releases the aminoacyl-tRNA which binds tightly to the A site
5. Peptidyltrasnferase reaction- peptide bond formed between the Meti at the P site and the second amino acid at the A site
6. This reaction is catalyzed by the large rRNA
7. EF2:GTP is hydrolyzed to EF2:GDP and the ribosome moves downstream by exactly one codon
8. The tRNA that used to hold Meti is now in the E site. The A site is empty for a new aminoacyl-tRNA to bind

Termination of Translation

1. eRF1 has a similar shape to tRNAs and can bind in the A site when there is a stop codon
2. eRF3:GTP works with eRF1 to cleave the peptide chain.
3. ABCE1 separates the subunits and releases the tRNA in the P site and the mRNA.
4. The small 40S subunit can bind with eIF1, 1A and 3 again for another round of translation. The mRNA can also be translated again.